Process of making low-carbon ferro-alloys



Patented Feb. 25, 1930 UNITED STATES PATENT orl-lcs FREDERICK I. BECKET,OF NEW YORK, N. Y., ASSIGNOR TO ELEC'IRO METALLURGICAL COMPANY, ACORPORATION OF WEST VIRGINIA.

PROCESS OF MAKING LOW-CARBON FERRO-ALLOYS 1T0 Drawing.

This invention relates to the art of making low-carbon ferro-alloystypified by rustless iron, low-carbon term-manganese, etc. The object ofthe invention is to provide a 5 novel and economical process forpreparing such alloys.

Rustless iron for the purposes of this invention" may be defined as analloy consisting essentially of iron and chromium, with upward of about9 percent of chromium and less than 0.2 percent of carbon. Nickel mayalso be, present in varying proportions replacing the-iron. A preferredcomposition contains about 12-14 percent of chromium 15 with 0.12percent or less of carbon. Another excellent composition contains around16-18 percent of chromium, with 7 -8 percent nickel and carbon around0.12 percent or less.

Such alloys have heretofore been made by 20 various processes, such asthe introduction of low-carbon ferrochrome or chromium metal into a bathof low-carbon iron; or by subjecting an iron-chromium alloy containingmore than'the desired proportion of carbon to an oxidizing treatment toeliminate a portion of the carbon, and thereafter deoxidizing theresulting low-carbon product, as disclosed in U. S. Patent 1,365,091 toClement.

It has likewise been proposed to reduce the carbon content offerrochromium by blowing the molten alloy with air or other oxidizinggas by a procedure somewhat analogous to the Bessemerization of steel,as described for instance in United States Patent 1,063,280

to Morehead and United States, Patent 1,063,341 to Edmands: but thisprocedure, if

continued until the carbon content is reduced to the very lowproportions herein contemplated, involves heavy losses of chromium,which oxidizes at increasing rate as the carbon content of: the moltenbath is diminished.

It has likewise been proposed to subject molten ferrochrome, at atemperature .of

about 15001600 C. to the action of a mixture of steam and hydrogen, towhich mixture other gases such as carbon dioxid, carbon 1 nonoxid or airmay be added, it being considered that the hydrogen or other re-Application filed February 25, 1927. Serial No. 171,078.

ducing gas would exert a restraining action on the oxidation of themetals of the molten alloy. However both hydrogen and steam exert astrong cooling effect upon molten ferrochrome, so that suitableoperating conditions are not readily maintained. 1

Hydrogen is known to be effective to some degree to remove carbon frommetals in the sol1d state under certain conditions, but attempts toreduce the carbon content of molten ferrochrome from sa 2.0 to less than0.2 percent by means of ydrogen, without application of external heat,were not successful: when the hydrogen was blown beneath the surface ofthe melt the latter quickly solidified, the reaction heat beinginsuificient to maintain fluidity; and introduced above the surface itproved quite ineifective to remove the carbon. On the other hand carboncan be quickly removed by oxygen, intromaintained or even increased bythe strong- 1y exothermic reaction: and thereafter or 1n some casessimultaneously the highly heated metal is blown with hydrogen to assist,in reducing its carbon content. By proceeding in this way the heatdeveloped during the oxygen blow is availed of to keep the metal moltenduring the hydrogen blow: and by properly controlling the respectiveblows a product of almost any desired carbon content down to a fewhundredths of one percent can be prepared. The use of hydrogen to efiectthe final decarburization to the desired limit presents the greatindustrial advantage, as compared with the use of oxygen for the samepurpose, that the elimination of carbon is accomplished with relativelysmall losses of chromium.

Following is an illustrative example of a procedure in accordance withmy invention,

it bein clearly understood that the invention is not limited to thetreatment of term alloys likewise oxidized during this blow, and thehighly exothermic reactions (oxidation of carbon, chromium, iron, andsilicon 1f present) result in a very highly heated and fimd melt. Forconvenience I will refer to this highly heated melt as superheated. Theoxygen is then cut off and hydrogen immediately introduced beneath thesurface of the superheated metal, and this is blown, (under fallingtemperature conditions) until the carbon is further reduced to thedesired point, for example to 0.10 or 0.12% or lower.

Certain other typical and specific instances to which however theinvention is by no means limited, are as follows:

A heat containing 19.36% Cr and 1.78% C was blown with pure oxygen, andyielded an alloy containing 16.03% Cr and .08% C, the chromium recoverybein 73.5%. A second heat containing 16.97% r and 1.15% C was similarlyblown with oxygen, yielding an alloy containing 12.93% Cr and 0.06% C,with a chromium recovery of 70.8%. These are typical of the resultsobtainable with pure ox n.

This average of two runs employing oxygen and hydrogen successively wasas follows:

The alloy treated contained 17.90% Cr with 2.04% C. At the conclusion ofthe oxygen blow its composition was 15.75% Cr and 0.17% C. This was thenblown with hydrogen and yielded an alloy containing 15.75% Cr and .05%C. The total chromium recovery was 84%.

In another instance the compositions of the original charge, and of thealloy after the oxygen and hydrogen blows respectively were as follows:

Alter After orlgmal oxygen hydrogen Per cm! Per cent Per cent Chromium21. 47 8. 53 18. 84 Ni 7. 64 a 24 8. 27 2. 01 0. 16 0. 10 0. 12

The total recovery of chromium was in this case approximately 80% and ofnickel approximately 98%.

Since the losses of chromium occur mainly during the oxygen blow it isof course advantageous, from the point of view of chromium recovery, toarrest the oxygen blow at a somewhat earlier point than was here thecase, say when the carbon content is about 0.2 to 0.3% ,having dueregard to the necessity above explained of a sufiicient superheat underthe particular operating conditlons to permit the subsequent hydrogenblow to be continued until the desired final carbon specification isattained.

External heat might of course be applied during the hydrogen blow toassist in maintaining the necessary temperature, and under theseconditions it would not be essential to superheat the metal b an oxygenblow in order to provide suitable conditions for the hydrogenblow. Ibelieve myself the first to accomplish the substantial decarburizationof a molten ferro alloy by means of hydrogen, and this inventioncontemplates such decarburization broadly, the term decarburizationbeing herein used to indicate a reduction of the carbon content to apoint not substantially in excess of about 0.2 percent, or in otherwords, to a point within the permissible rustless iron range.

The general principle involved in this invention in its preferredembodiment is the use of an oxygen (pure or diluted) blast to maintain,and in part at least to provide, the very high temperature of the meltwhich is essential to efficient decarburization by hydrogen, thedecarburizing action of oxygen being supplemented by that of hydrogen.Within this principle the procedure may be variously modified. Forinstance the hydrogen and oxygen may be introduced simultaneously,although preferably not in admixture with each other, the oxygen beingpreferably supplied above the surface of the inelt and the hydrogenbelow the surface. The ratio of oxygen and hydrogen may be varied as theblow pro-.

gresses to maintain any desired temperature conditions. One verydesirable modlfication is to blow first with oxygen, applied above thesurface, and then to continue the treatment with both oxygen andhydrogen, applied respectively above and below the surface of the metal.

The processes as described are applicable with appropriate modificationsto the manufacture of low-carbon ferroalloys other than rustless iron;and in particular to the manu facture of low-carbon ferromanganese.

Certain alloys, including iron-chromium alloys, after treatment withhydrogen as described herein exhibit a porosity due to blowholes. Wherethis effect is so pronounced as to be objectionable it may be overcomeby treatment with appropriate reagents, among which I now prefer to usea lead-sodium alloy, alone or in conjunction with the usual finishingreagents such as silicon and manganese; or a solid metal may be obtainedby subjecting the melt to a very short or indeed momentary blow withoxygen, following the hydrogen blow, in which case it is desirable tofollow this by the well known deoxidizing treatments. Other treatmentsof like efiect ma of course be used.

claim:

1. The process of decarburizing ferroalloys which comprises blowing themolten alloy with chemically uncombined oxygen to remove carbon andraise the temperature of the alloy; blowing hydrogen, substantially freefrom admixed oxygen compounds through the molten alloy to removeadditional uantities of carbon; and continuing the hy rogen blast afterthe carbon content is reduced below 0.2%.

2. The process of making rustless iron from an iron-chromium alloycontaining excess carbon which comprises blowing the mol- 0 tenalloywith chemically uncombined oxygen to remove carbon and raise thetemperature of the alloy; blowing hydrogen, substantiallyfree fromadmixed oxygen compounds, through the molten alloy to remove additionaluantities of carbon; and continuing the hy rogen blast after the carboncontent is reduced below 0.2%. v

3. The process of making rustless iron from an iron-chromium alloycontaining ex- 80 cess carbon which comprises blowing the molten alloywith a gas containing uncombined oxygen in a higher proportion than ispresent in air to remove carbon and raise the temerature of the alloy;and then removing a 5 rther quantity of carbon by blowing w1th hydrogenin theabsence of substantial quantities of gaseous oxygen-containingsubstances, the carbon content during the latter part of the hydrogenblowbeing less than 4. The process of making rustless iron from aniron-chromium alloy containing excess carbon which comprises blowingoxygen onto the surface of the molten alloy to raise its temperature andremove carbon; and then blowing hydrogen substantially free from gaseousoxygen containing substances through the molten alloy to remove morecarbon, the carbon content of the alloy being below 0.2% during thelatter part of the hydrogen blow.

In testimony whereof, I afiix my signature.

' FREDERICK M. .BECKET.

